We propose a comparative study of transformation-specific genes and their products in the class of defective, rapidly transforming avian retroviruses. Our work will concentrate on several new virus isolates from spontaneous and chemically induced chicken tumors with the following specific aims: 1. Biological characterization of the new viruses will include tumor spectra, defectiveness of replication and properties of the transformed cells. 2. Nucleic acid studies will define transformation-related sequences with specific cDNA probes, RNase-T, fingerprints and heteroduplex mapping in the electron microscope. 3. Transformation-specific proteins will be characterized by two-dimensional tryptic peptide analysis, in vitro translation, tests for phosphokinase activity and reactivity with monoclonal antibodies. 4. Conditional and nonconditional mutants affecting the transforming ability of the new viruses will be sought. We expect that these studies will reveal common features of the transformation-related proteins in several groups of avian retroviruses and that the results will advance our knowledge of viral oncogenesis.